This invention relates to a drive circuit for a brushless (BL) motor. More particularly, this invention relates to a drive circuit for a BL motor utilizing Hall elements to reduce acoustic noises during phase switching. Still more particularly, this invention relates to a drive circuit for a conventional BL motor employing Hall elements for producing output energization signals having a predetermined desirable waveform for application to stator coils on the BL motor.
A number of switching drive systems for energizing conventional BL motors are known. Such a switching drive system sometimes utilizes switching transistors for switching the energization or phase for such a conventional BL motor. When using such a switching drive system, a resulting high-frequency mechanical vibration of stators and rotors of the BL motor generates relatively loud acoustic noises which are caused by abrupt current changes at the time of phase switching. The acoustic noises are most noticeable when caused by the changes occurring at the rising edges of the applied motor current. Conventionally, condensers have been used to prevent the generation of the acoustic noises. The condensers, such as electrolytic condensers, having a comparatively large capacity, are connected in parallel with the stator coils to slow current changes during phase switching.
The conventional method using such condensers has disadvantages. For example, the method is not effective when the rotation number of the motor is small. Furthermore, use of condensers degrades motor efficiency because of the phase delay of an energization current which occurs which in turn causes an invalid current to flow when the rotation number of the motor is large.
A linear drive system for a BL motor is also known which utilizes a sinusoidal energization current. While such a circuit is effective in reducing acoustic noise, the motor drive efficiency is remarkably lowered by such a linear drive system compared with a switching drive system.
Accordingly, it has remained a problem in the art to reduce a motor drive circuit for a BL motor which maintains motor efficiency comparable with that of a switching drive system, while reducing acoustic noises during phase switching.
Thus, it is an overall object of the invention, therefore, to provide a BL motor drive circuit having a motor efficiency which is comparable to the efficiency of a switching drive system, while reducing acoustic noises during phase switching.
It is another general object of this invention to provide a BL motor drive circuit to achieve those results by replacement of the drive circuit, without altering the mechanical structure of an conventional BL motor.
It is an additional object of this invention to provide a BL motor drive circuit using output signals of Hall elements for position detection to produce waveforms having predetermined characteristics to prevent acoustic noises during switching while preserving the efficiency of the switching drive system.
These and other objects and features of the present invention will become apparent from the foregoing detailed description of the invention and the appended claims, with reference to the accompanying drawings.
BRIEF SUMMARY OF THE INVENTION
Directed to overcoming the problems noted above with respect to the prior art and achieving the above-stated objects, the invention relates to a BL motor with a three-phase bi-directional, 120.degree. energization system, by way of example. According to the invention, output signals from a plurality of Hall elements are provided for detecting the position of the rotor and providing an energization signal through accompanying circuitry having edge portions similar to inclined waveforms of the edge portion of the output signals. The energization signals are supplied to stator coils of the BL motor, whereby acoustic noise generation during switching is prevented with the same efficiency as the switching drive system.
The waveforms of the Hall elements depend upon magnetic flux changes from rotor magnets in the BL motor. Thus, the waveforms of the output signals of the Hall elements are almost trapezoidal, depending on the distances between the rotor magnets and the Hall elements, the positions or sizes of the non-magnetic regions of the rotor magnets, and other factors. According to the invention, the edge portions of the energization signals are formed by shifting the level or amplifying the edge portions at the output signals of the Hall elements to provide the desired waveform. Accordingly, the edge portions of the energization signals do not become abrupt and thus prevent the generation of acoustic noises. Because the output waveforms are utilized according to the invention, no phase change occurs even if the rotation number of the BL motor varies. Moreover, since a trapezoidal drive signal is thus employed, motor efficiency is not lowered.
According to one aspect of the invention, a BL motor drive circuit according to the invention comprises a plurality of Hall elements for detecting magnetic flux changes in a rotating rotor magnet. A drive signal generating circuit is connected to the Hall elements for generating multi-phase drive signals which are supplied to the stator windings of a multi-phase BL motor. Each of the multi-phase drive signals has a pair of slanted edge portions and a relatively flat portion interposed between the pair of slanted edge portions which are directly formed from the detected flux changes. Thus, the drive signal is generally trapezoidal in shape. Preferably, the drive signal generating circuit includes a signal clamping circuit to form the flat portion of the generally trapezoidal multi-phase drive signal, while a portion of the slanted edge portions of adjacent pairs of the multi-phase drive signals form an overlapping drive area. The slanted edge portion of the multi-phase drive signal subtends an electrical angle which lies within the range of 2.degree. el. to 30.degree. el.
In a typical operative embodiment, the drive signal generating circuit includes a plurality of buffer amplifying circuits connected to the Hall elements; clamping circuits; non-adder mixing circuits; analog switching circuits; and output amplifiers respectively connected in series, a plurality of such series circuits being respectively connected in parallel with the plurality of Hall elements. Each drive signal generating circuit further includes swithcing pulse generating circuits for generating switching pulses supplied to the analog switching circuts based upon detected magnetic flux changes. In an alternative embodiment, non-adder mixing circuits are replaced by signal mixing circuits in the series arrangements. These and other features of the invention will become apparent from a review of the written description of the invention which follows, taken in conjunction with the accompanying drawings.